:root {
    --bg-primary: #ffffff;
    --bg-secondary: #f5f5f5;
    --bg-tertiary: #e5e5e5;
    --text-primary: #1a1a1a;
    --text-secondary: #666666;
    --accent: #2563eb;
    --accent-hover: #1d4ed8;
    --border: #d1d5db;
    --success: #10b981;
    --warning: #f59e0b;
    --error: #ef4444;
    --shadow: rgba(0, 0, 0, 0.1);
}

[data-theme="dark"] {
    --bg-primary: #1a1a1a;
    --bg-secondary: #2d2d2d;
    --bg-tertiary: #404040;
    --text-primary: #e5e5e5;
    --text-secondary: #a3a3a3;
    --accent: #3b82f6;
    --accent-hover: #60a5fa;
    --border: #404040;
    --success: #34d399;
    --warning: #fbbf24;
    --error: #f87171;
    --shadow: rgba(0, 0, 0, 0.3);
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    transition: background 0.2s, color 0.2s;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 1rem;
}

.navbar {
    background: var(--bg-secondary);
    border-bottom: 1px solid var(--border);
    padding: 0.75rem 0;
    position: sticky;
    top: 0;
    z-index: 100;
}

.navbar .container {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.nav-brand {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--text-primary);
    text-decoration: none;
}

.nav-links {
    display: flex;
    gap: 1.5rem;
    align-items: center;
}

.nav-link {
    color: var(--text-secondary);
    text-decoration: none;
    transition: color 0.2s;
}

.nav-link:hover {
    color: var(--accent);
}

main {
    padding: 2rem 0;
    min-height: calc(100vh - 4rem);
}

h1, h2, h3, h4 {
    margin-bottom: 1rem;
    font-weight: 600;
}

h1 { font-size: 2rem; }
h2 { font-size: 1.5rem; }
h3 { font-size: 1.25rem; }

.btn {
    display: inline-block;
    padding: 0.5rem 1rem;
    background: var(--accent);
    color: white;
    border: none;
    border-radius: 0.375rem;
    cursor: pointer;
    font-size: 0.875rem;
    font-weight: 500;
    text-decoration: none;
    transition: background 0.2s;
}

.btn:hover {
    background: var(--accent-hover);
}

.btn-secondary {
    background: var(--bg-tertiary);
    color: var(--text-primary);
}

.btn-secondary:hover {
    background: var(--border);
}

.btn-sm {
    padding: 0.375rem 0.75rem;
    font-size: 0.75rem;
}

.icon-btn {
    background: none;
    border: none;
    cursor: pointer;
    font-size: 1.25rem;
    padding: 0.25rem;
    color: var(--text-primary);
    transition: opacity 0.2s;
}

.icon-btn:hover {
    opacity: 0.7;
}

.card {
    background: var(--bg-secondary);
    border: 1px solid var(--border);
    border-radius: 0.5rem;
    padding: 1.5rem;
    margin-bottom: 1rem;
}

.card-title {
    font-size: 1.125rem;
    font-weight: 600;
    margin-bottom: 0.5rem;
}

.stats-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 1rem;
    margin-bottom: 2rem;
}

.stat-card {
    background: var(--bg-secondary);
    border: 1px solid var(--border);
    border-radius: 0.5rem;
    padding: 1.5rem;
    text-align: center;
}

.stat-value {
    font-size: 2rem;
    font-weight: 700;
    color: var(--accent);
    display: block;
}

.stat-label {
    font-size: 0.875rem;
    color: var(--text-secondary);
    margin-top: 0.25rem;
}

.progress-bar {
    width: 100%;
    height: 0.5rem;
    background: var(--bg-tertiary);
    border-radius: 0.25rem;
    overflow: hidden;
    margin: 0.5rem 0;
}

.progress-fill {
    height: 100%;
    background: var(--accent);
    transition: width 0.3s;
}

.unit-list {
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.unit-card {
    background: var(--bg-secondary);
    border: 1px solid var(--border);
    border-radius: 0.5rem;
    padding: 1.5rem;
    text-decoration: none;
    color: var(--text-primary);
    transition: border-color 0.2s, box-shadow 0.2s;
}

.unit-card:hover {
    border-color: var(--accent);
    box-shadow: 0 4px 6px var(--shadow);
}

.unit-header {
    display: flex;
    justify-content: space-between;
    align-items: start;
    margin-bottom: 0.75rem;
}

.unit-title {
    font-size: 1.125rem;
    font-weight: 600;
}

.unit-badge {
    background: var(--accent);
    color: white;
    padding: 0.25rem 0.5rem;
    border-radius: 0.25rem;
    font-size: 0.75rem;
}

.unit-summary {
    color: var(--text-secondary);
    font-size: 0.875rem;
    margin-bottom: 0.75rem;
}

.unit-meta {
    display: flex;
    gap: 1rem;
    font-size: 0.75rem;
    color: var(--text-secondary);
}

.prerequisites {
    margin-top: 0.5rem;
    padding-top: 0.5rem;
    border-top: 1px solid var(--border);
    font-size: 0.75rem;
    color: var(--text-secondary);
}

.lesson-content {
    max-width: 800px;
    margin: 0 auto;
    line-height: 1.8;
}

.lesson-content h2 {
    margin-top: 2rem;
    padding-bottom: 0.5rem;
    border-bottom: 1px solid var(--border);
}

.lesson-content p {
    margin-bottom: 1rem;
}

.lesson-content ul, .lesson-content ol {
    margin-bottom: 1rem;
    padding-left: 2rem;
}

.lesson-content li {
    margin-bottom: 0.5rem;
}

.lesson-content code {
    background: var(--bg-tertiary);
    padding: 0.125rem 0.25rem;
    border-radius: 0.25rem;
    font-size: 0.875em;
    font-family: 'Courier New', monospace;
}

.lesson-content pre {
    background: var(--bg-tertiary);
    padding: 1rem;
    border-radius: 0.375rem;
    overflow-x: auto;
    margin-bottom: 1rem;
}

.lesson-content pre code {
    background: none;
    padding: 0;
}

.lesson-content table {
    width: 100%;
    border-collapse: collapse;
    margin-bottom: 1rem;
}

.lesson-content th,
.lesson-content td {
    border: 1px solid var(--border);
    padding: 0.5rem;
    text-align: left;
}

.lesson-content th {
    background: var(--bg-secondary);
    font-weight: 600;
}

.lesson-content strong {
    font-weight: 600;
}

.lesson-content sup {
    font-size: 0.75em;
    vertical-align: super;
}

.flashcard {
    max-width: 600px;
    margin: 2rem auto;
    perspective: 1000px;
}

.flashcard-inner {
    position: relative;
    width: 100%;
    min-height: 300px;
    transition: transform 0.6s;
    transform-style: preserve-3d;
}

.flashcard-inner.flipped {
    transform: rotateY(180deg);
}

.flashcard-face {
    position: absolute;
    width: 100%;
    min-height: 300px;
    backface-visibility: hidden;
    background: var(--bg-secondary);
    border: 2px solid var(--border);
    border-radius: 0.5rem;
    padding: 2rem;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    text-align: center;
}

.flashcard-back {
    transform: rotateY(180deg);
}

.flashcard-text {
    font-size: 1.25rem;
    line-height: 1.6;
}

.flashcard-label {
    font-size: 0.75rem;
    text-transform: uppercase;
    color: var(--text-secondary);
    margin-bottom: 1rem;
}

.flashcard-controls {
    max-width: 600px;
    margin: 2rem auto;
    text-align: center;
}

.flashcard-info {
    display: flex;
    justify-content: space-between;
    margin-bottom: 1rem;
    font-size: 0.875rem;
    color: var(--text-secondary);
}

.grade-buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 0.5rem;
    margin-top: 1rem;
}

.grade-btn {
    padding: 1rem;
    font-size: 0.875rem;
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 0.25rem;
}

.grade-btn .key {
    font-size: 0.75rem;
    opacity: 0.7;
}

.quiz-question {
    max-width: 800px;
    margin: 0 auto;
}

.quiz-progress {
    margin-bottom: 2rem;
}

.question-text {
    font-size: 1.25rem;
    margin-bottom: 1.5rem;
}

.choices {
    display: flex;
    flex-direction: column;
    gap: 0.75rem;
}

.choice {
    padding: 1rem;
    background: var(--bg-secondary);
    border: 2px solid var(--border);
    border-radius: 0.375rem;
    cursor: pointer;
    transition: border-color 0.2s, background 0.2s;
}

.choice:hover {
    border-color: var(--accent);
}

.choice.selected {
    border-color: var(--accent);
    background: var(--bg-tertiary);
}

.choice.correct {
    border-color: var(--success);
    background: rgba(16, 185, 129, 0.1);
}

.choice.incorrect {
    border-color: var(--error);
    background: rgba(239, 68, 68, 0.1);
}

.explanation {
    margin-top: 1.5rem;
    padding: 1rem;
    background: var(--bg-secondary);
    border-left: 4px solid var(--accent);
    border-radius: 0.375rem;
}

.explanation-label {
    font-weight: 600;
    margin-bottom: 0.5rem;
}

.match-game {
    max-width: 900px;
    margin: 0 auto;
}

.match-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 2rem;
}

.match-timer {
    font-size: 1.5rem;
    font-weight: 600;
    color: var(--accent);
}

.match-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
    gap: 0.75rem;
}

.match-card {
    padding: 1rem;
    background: var(--bg-secondary);
    border: 2px solid var(--border);
    border-radius: 0.375rem;
    cursor: pointer;
    text-align: center;
    transition: all 0.2s;
    min-height: 80px;
    display: flex;
    align-items: center;
    justify-content: center;
}

.match-card:hover {
    border-color: var(--accent);
    transform: translateY(-2px);
}

.match-card.selected {
    border-color: var(--accent);
    background: var(--bg-tertiary);
}

.match-card.matched {
    opacity: 0.5;
    cursor: default;
    border-color: var(--success);
}

.match-card.matched:hover {
    transform: none;
}

.action-buttons {
    display: flex;
    gap: 0.5rem;
    margin-top: 1rem;
    flex-wrap: wrap;
}

.empty-state {
    text-align: center;
    padding: 4rem 2rem;
    color: var(--text-secondary);
}

.empty-state-icon {
    font-size: 3rem;
    margin-bottom: 1rem;
}

.input-group {
    margin-bottom: 1rem;
}

.input-group label {
    display: block;
    margin-bottom: 0.5rem;
    font-weight: 500;
    font-size: 0.875rem;
}

.input-group input,
.input-group select {
    width: 100%;
    padding: 0.5rem;
    background: var(--bg-primary);
    border: 1px solid var(--border);
    border-radius: 0.375rem;
    color: var(--text-primary);
    font-size: 0.875rem;
}

.modal {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0, 0, 0, 0.5);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 1000;
    padding: 1rem;
}

.modal-content {
    background: var(--bg-primary);
    border-radius: 0.5rem;
    padding: 2rem;
    max-width: 500px;
    width: 100%;
    max-height: 90vh;
    overflow-y: auto;
}

.modal-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1.5rem;
}

.modal-title {
    font-size: 1.25rem;
    font-weight: 600;
}

.close-btn {
    background: none;
    border: none;
    font-size: 1.5rem;
    cursor: pointer;
    color: var(--text-secondary);
    padding: 0;
    line-height: 1;
}

.alert {
    padding: 0.75rem 1rem;
    border-radius: 0.375rem;
    margin-bottom: 1rem;
}

.alert-success {
    background: rgba(16, 185, 129, 0.1);
    border: 1px solid var(--success);
    color: var(--success);
}

.alert-error {
    background: rgba(239, 68, 68, 0.1);
    border: 1px solid var(--error);
    color: var(--error);
}

@media (max-width: 768px) {
    .nav-links {
        gap: 0.75rem;
    }
    
    .nav-link {
        font-size: 0.875rem;
    }
    
    h1 { font-size: 1.5rem; }
    h2 { font-size: 1.25rem; }
    
    .stats-grid {
        grid-template-columns: repeat(2, 1fr);
    }
    
    .flashcard-face {
        min-height: 250px;
        padding: 1.5rem;
    }
    
    .flashcard-text {
        font-size: 1.125rem;
    }
    
    .grade-buttons {
        grid-template-columns: repeat(2, 1fr);
    }
    
    .match-grid {
        grid-template-columns: repeat(auto-fill, minmax(150px, 1fr));
    }
}

@media (max-width: 480px) {
    .stats-grid {
        grid-template-columns: 1fr;
    }
    
    .unit-header {
        flex-direction: column;
        gap: 0.5rem;
    }
    
    .match-grid {
        grid-template-columns: 1fr;
    }
}